Fix regression caused by #13573 (#13600)

Broke in #13573.

changelog.d/13600.misc

@@ -0,0 +1 @@
+Cache user IDs instead of profiles to reduce cache memory usage. Contributed by Nick @ Beeper (@fizzadar).

synapse/storage/databases/main/roommember.py

@@ -916,7 +916,9 @@ class RoomMemberWorkerStore(EventsWorkerStore):
             event_to_memberships = await self._get_user_ids_from_membership_event_ids(
                 missing_member_event_ids
             )
-            users_in_room.update(event_to_memberships.values())
+            users_in_room.update(
+                user_id for user_id in event_to_memberships.values() if user_id
+            )
 
         if event is not None and event.type == EventTypes.Member:
             if event.membership == Membership.JOIN:
@@ -942,15 +944,15 @@ class RoomMemberWorkerStore(EventsWorkerStore):
     )
     async def _get_user_ids_from_membership_event_ids(
         self, event_ids: Iterable[str]
-    ) -> Dict[str, str]:
+    ) -> Dict[str, Optional[str]]:
         """For given set of member event_ids check if they point to a join
-        event and if so return the associated user and profile info.
+        event.
 
         Args:
             event_ids: The member event IDs to lookup
 
         Returns:
-            Map from event ID to `user_id` and ProfileInfo (or None if not join event).
+            Map from event ID to `user_id`, or None if event is not a join.
         """
 
         rows = await self.db_pool.simple_select_many_batch(